Angiogenesis, the sprouting of new blood vessels from the pre-existing vasculature, plays an important role in a wide range of physiological and pathological processes (Nguyen, L. L. et al, Int. Rev. Cytol., 204, 1-48, (2001)). Angiogenesis is a complex process, mediated by communication between the endothelial cells that line blood vessels and their surrounding environment. In the early stages of angiogenesis, tissue or tumor cells produce and secrete pro-angiogenic growth factors in response to environmental stimuli such as hypoxia. These factors diffuse to nearby endothelial cells and stimulate receptors that lead to the production and secretion of proteases that degrade the surrounding extracellular matrix. The activated endothelial cells begin to migrate and proliferate into the surrounding tissue toward the source of these growth factors (Bussolino, F., Trends Biochem. Sci., 22, 251-256, (1997)). Endothelial cells then stop proliferating and differentiate into tubular structures, which is the first step in the formation of stable, mature blood vessels. Subsequently, periendothelial cells, such as pericytes and smooth muscle cells, are recruited to the newly formed vessel in a further step toward vessel maturation.
Angiogenesis is regulated by a balance of naturally occurring pro- and anti-angiogenic factors. Vascular endothelial growth factor, fibroblast growth factor, and angiopoeitin represent a few of the many potential pro-angiogenic growth factors. These ligands bind to their respective receptor tyrosine kinases on the endothelial cell surface and transduce signals that promote cell migration and proliferation. Whereas many regulatory factors have been identified, the molecular mechanisms that drive this process are still not fully understood.
There are many disease states driven by persistent unregulated or improperly regulated angiogenesis. In such disease states, unregulated or improperly regulated angiogenesis may either cause a particular disease or exacerbate an existing pathological condition. For example, ocular neovascularization has been implicated as the most common cause of blindness and underlies the pathology of approximately 20 eye diseases. In certain previously existing conditions, such as arthritis, newly formed capillary blood vessels invade the joints and destroy cartilage. In diabetes, new capillaries formed in the retina invade the vitreous humor, causing bleeding and blindness.
Both the growth and metastasis of solid tumors may also be angiogenesis-dependent, Folkman et al., “Tumor Angiogenesis,” Chapter 10, 206-32, in The Molecular Basis of Cancer, Mendelsohn et al., eds., W. B. Saunders, (1995). It has been shown that tumors which enlarge to greater than 2 mm in diameter must obtain their own blood supply and do so by inducing the growth of new capillary blood vessels. After these new blood vessels become embedded in the tumor, they provide nutrients and growth factors essential for tumor growth as well as a means for tumor cells to enter the circulation and metastasize to distant sites, such as liver, lung or bone (Weidner, New Eng. J. Med., 324, 1, 1-8 (1991)). When used as drugs in tumor-bearing animals, natural inhibitors of angiogenesis may prevent the growth of small tumors (O'Reilly et al., Cell, 79, 315-28 (1994)). In some protocols, the application of such inhibitors leads to tumor regression and dormancy even after cessation of treatment (O'Reilly et al., Cell, 88, 277-85 (1997)). Moreover, supplying inhibitors of angiogenesis to certain tumors may potentiate their response to other therapeutic regimens (see, e.g., Teischer et al., Int. J. Cancer, 57, 920-25 (1994)).
Although many disease states are driven by persistent unregulated or improperly regulated angiogenesis, some disease states may be treated by enhancing angiogenesis. Tissue growth and repair are biologic events wherein cellular proliferation and angiogenesis occur. Thus an important aspect of wound repair is the revascularization of damaged tissue by angiogenesis.
Chronic, non-healing wounds are a major cause of prolonged morbidity in the aged human population. This is especially the case in bedridden or diabetic patients who develop severe, non-healing skin ulcers. In many of these cases, the delay in healing is a result of inadequate blood supply either as a result of continuous pressure or of vascular blockage. Poor capillary circulation due to small artery atherosclerosis or venous stasis contributes to the failure to repair damaged tissue. Such tissues are often infected with microorganisms that proliferate unchallenged by the innate defense systems of the body which require well vascularized tissue to effectively eliminate pathogenic organisms. As a result, most therapeutic intervention centers on restoring blood flow to ischemic tissues thereby allowing nutrients and immunological factors access to the site of the wound.
Atherosclerotic lesions in large vessels may cause tissue ischemia that could be ameliorated by modulating blood vessel growth to the affected tissue. For example, atherosclerotic lesions in the coronary arteries may cause angina and myocardial infarction that could be prevented if one could restore blood flow by stimulating the growth of collateral arteries. Similarly, atherosclerotic lesions in the large arteries that supply the legs may cause ischemia in the skeletal muscle that limits mobility and in some cases necessitates amputation, which may also be prevented by improving blood flow with angiogenic therapy.
Other diseases such as diabetes and hypertension are characterized by a decrease in the number and density of small blood vessels such as arterioles and capillaries. These small blood vessels are important for the delivery of oxygen and nutrients. A decrease in the number and density of these vessels contributes to the adverse consequences of hypertension and diabetes including claudication, ischemic ulcers, accelerated hypertension, and renal failure. These common disorders and many other less common ailments, such as Burgers disease, could be ameliorated by increasing the number and density of small blood vessels using angiogenic therapy.
Thus, there is a continuing need to identify regulators of angiogenesis.
In view of the foregoing, there is a need to identify biochemical targets in the treatment of angiogenesis mediated disorders. However, angiogenesis involves the action of multiple growth factors and their cognate receptor tyrosine kinases (RTKs), Yancopoulos et al., Nature, 407,242-248, 2000). Vascular endothelial growth factor (VEGF), for example, is important for the differentiation of endothelial cells into nascent blood vessels in the embryonic vasculature. Further, VEGF enhances blood vessel development in the adult vasculature. Administration of exogenous VEGF enhances the development of the collateral vasculature and improves blood flow to ischemic tissues.
To date, three VEGF RTKs have been identified, VEGFR1 (FLT-1), VEGFR2 (KDR), and VEGFR3 (FLT-4). Although these receptors are highly conserved, based on biochemical characterization and biological activity, each has specific and non-overlapping functions. Of the three receptors, VEGFR2 is believed to play the predominant role in mediating VEGF actions in the developing vasculature and during angiogenesis in adults. However, both VEGFR1 and VEGFR3 are required for normal development of the embryonic vasculature and may also be important for angiogenesis in adult tissues. Upon VEGF binding and dimerization, a conformational change in the VEGFR2 kinase domain enhances its kinase activity resulting in “autophosphorylation” of the other member of the pair on specific tyrosine residues. These autophosphorylation events serve to further enhance the kinase activity and provide anchor points for the association of intracellular signaling molecules.
However, activation of a single angiogenic pathway may not be sufficient to produce persistent and functional vessels that provide adequate perfusion to ischemic tissue. These findings, together with fact that multiple RTKs are involved in the assembly of embryonic vasculature, indicate that biochemical targets that modulate multiple angiogenic pathways will have advantages over administration of a single growth factor.
Protein tyrosine phosphatases (PTPs) comprise a large family of closely related enzymes that dephosphorylate proteins that contain phosphotyrosine residues. Recent evidence suggests that one function of PTPs is to limit the phosphorylation and activation of RTKs. For example, HCPTPA, a low molecular weight protein tyrosine phosphatase, was shown to associate with VEGFR2 and negatively regulate its activation in cultured endothelial cells and its biological activity in angiogenesis assays, (Huang et al., Journal of Biological Chemistry, 274, 38183-38185, 1999).
In addition to VEGFR2, signaling input from another RTK, Tie-2, the receptor for the angiopoietins (Ang1 and Ang2), is also important. Deletion of either the Ang1 or Tie-2 gene in mice may result in embryonic lethality secondary to abnormalities in the developing vasculature (Yancopoulos et al., Nature, 407, 242-248, 2000). In addition, overexpression of Ang1 in the skin increases skin vascularity and administration of exogenous Ang1 increases blood flow to ischemic skeletal muscle (Suri et al., Science, 282, 468-471, 1998). Moreover, inhibiting the activation of Tie-2 inhibits angiogenesis and limits tumor progression in animal models of cancer, (Lin et al., J Clin. Invest., 100, 2072-2078, 1997). In addition to its angiogenic activities, activation of Tie-2 by exogenous administration of Ang1 blocks VEGF mediated vascular leak and pro-inflammatory effects, but enhances its angiogenic effects (Thurston et al., Nature Medicine, 6, 460-463, 2000). Therefore, biological targets that modulate both VEGFR2 and Tie-2 signaling may yield superior proangiogenic or antiangiogenic therapies.
HPTPbeta (first described in Kruegar et al., EMBO J., 9, (1990)) has been suggested for modulating the activity of angiopoietin receptor-type tyrosine kinase Tie-2, e.g., WO 00/65088). HPTPbeta is also suggested for regulating activities of VEGFR2, e.g., US Pat. Pub. No. 2004/0077065.
It would be desirable to develop antibodies, e.g., a humanized monoclonal antibody, which selectively regulate the activity of HPTPbeta and thereby enhance angiogenic signaling, stimulate blood vessel growth (angiogenesis), and/or increase blood flow in ischemic tissue, or reduce angiogenic signaling, reduce blood vessel growth, and/or decrease blood flow to the effected tissue. Herein are described antibodies and fragments thereof that bind HPTPbeta and regulate angiogenic cell signaling, which in turn, regulates angiogenesis.